1. Field of the Invention
The present invention relates to a method of measuring a gap between a mask and a substrate of flat panel displays, such as plasma display panels and liquid crystal displays.
2. Description of the Related Art
A photolithography process for providing patterns on substrates of flat panel displays is a part of the fabrication of the flat panel displays. The exposure phase of the photolithographic process is achieved by aligners.
Aligners for fabrication of flat panel displays often adopt a proximity exposure method. The proximity exposure method involves maintaining a small gap, 50 to 250 microns wide, between the substrate and the mask during exposure. This gap minimizes mask damage.
A conventional aligner for proximity exposure is disclosed in Japanese Unexamined Patent Application No. Jp-A 2001-12905. FIG. 1 shows a schematic of a conventional aligner and is designated by reference numeral 150. The aligner 150 includes a substrate stage 106 that has an upper surface 106S. A transparent substrate 104, such as a glass substrate, is disposed on the upper surface 106 and is secured by vacuum clamping. The upper surface 106S is square and has a side-length of L6.
The substrate stage 106 is connected to stage drivers 143, which are respectively controlled by controllers 140. The positional control of the substrate stage 106, including the vertical control and leveling, is achieved by controllers 140 and stage drivers 143.
As shown in FIG. 2, the substrate 104 is square and has a side-length of L4. Except for the reflective square regions 105 near the corners of the main surface of the substrate 104, the main surface of the substrate 104 is covered with a photo resist 104a (FIG. 1). That is, the substrate 104 is exposed at the square regions 105. The exposed square regions are referred to as gap measuring reflector regions 105, hereinafter. The gap measuring reflector regions 105 have a side-length of L5.
As shown in FIG. 1, the aligner 150 includes a frame-structured mask stage 103 onto which a square mask 101 having a side-length of L1 is secured. The mask 101 has a main surface, disposed opposite substrate 104, on which a transfer pattern is formed. As shown in FIG. 3, transparent gap measuring marks 102 are disposed near the respective corners of the mask 101. The gap measuring windows 102 are square and have a side-length of L2.
As shown in FIG. 1, the aligner 150 also includes laser beam emitters 107, such as laser diodes, and laser beam detectors 108, such as photo diodes. The laser beam emitters 107 and the laser beam detectors 108 are disposed over the mask 101. The laser beam emitters 107 project laser beams 109 onto the gap measuring windows 102 at an angle of 45 degree with respect to the mask 101. A part of each laser beam 109 is reflected by the mask 101 to generate a reflected beam 110 while the other part of the each laser beam 109 passes through the gap measuring windows 102 to generate a reflected beam 111. Each of the laser beam detectors 108 receives the reflected beam 110 from the mask 101 and the reflected beam 111 from the substrate 104.
The exposure process by the aligner 150 begins with positioning the mask 101 and the substrate 104 so that the centers of the windows 102 and the reflector regions 105 are aligned.
Then, the gaps between the mask 101 and the substrate 104 are measured at the corners with the laser beam emitters 107 and the laser beam detectors 108. The laser beam emitters 107 respectively project the laser beams 109 onto the gap measuring windows 102 at an angle of incident of 45 degrees. The laser beam detectors 108 receive the reflected beams 110 from the mask 101 and the reflected beams 111 from the substrate 104, and the laser beam detectors 108 generate spot position data representative of the positions of the spots where the laser beam detectors 108 receive the reflected beams 110 and 111. The spot position data may be representative of the distance between the spots of the reflected beams 110 and 111 provided on the laser beam detectors 108. Controllers 140 calculate the associated gaps between the mask 101 and the substrate 104, located near the corners, on the basis of the spot position data received from the receivers 108.
Controllers 140 then operate drivers 143 to control the position of the substrate stage 106 so that the gaps becomes equal.
After positioning substrate stage 106, the photo resist disposed on the substrate 104 is exposed with an ultraviolet light, which goes through the pattern on the mask 101.
However, the conventional aligner thus described suffers from a problem in that the pattern on the substrate requires reflective gap measuring marks. This undesirably reduces flexibility of the design of the pattern on the substrate.
An aligner that solves this problem is disclosed in Japanese Unexamined Patent Application No. Jp-A-Heisei 11-194501. This aligner is equipped with a substrate holder, a thickness measuring unit, a gap sensor and a controller. The substrate holder has an upper surface on which a substrate is secured. The thickness measuring unit measures the thickness of the substrate. The gap sensor determines the gap between the mask and the upper surface of the substrate holder. The controller calculates the gap between the mask and the substrate from the gap between the mask and the upper surface of the substrate holder and the thickness of the substrate, and the controller regulates the gap between the mask and the substrate in response to the calculated gap. This eliminates the need for providing reflective gap measuring marks on the substrate.
Another aligning method to achieve accurate alignment of the mask and the substrate is disclosed in Japanese Unexamined Patent Application No. Jp-A-Heisei 7-260424. This aligning method provides first alignment marks, consisting of diffraction gratings, on the mask at predetermined intervals, and also provides second alignment marks of diffraction gratings on the substrate. A laser beam emitted from a He—Ne laser is projected onto the mask and the substrate and is diffracted by the first and second alignment marks respectively disposed on the mask and the substrate. The relative position of the mask and the substrate is determined on the basis of the diffracted beams from the first and second alignment marks. Using diffracted beams enables accurate determination of the relative position. The mask and the substrate are then aligned in response to the determined relative position. An accurate determination of the relative position allows the mask and the substrate to be accurately aligned.
Recently, the size of flat display panel substrates have been enlarged to improve production efficiency. Substrates having a length of more than one meter, for example, are commercially available. Enlarging the substrates allows for a plurality of display device to be fabricated on a single substrate, thus, decreasing the number of required steps. For example, a large substrate, on which a plurality of display device have been fabricated, reduces the number of exposure processes necessary for fabricating the same number of display devices. This effectively reduces the fabrication cost of the display devices.
Enlarging the substrate, however, produces an undesirable deflection of the mask because enlargement of the substrate is inevitably accompanied by the enlargement of the mask used in the exposure process. Any deflection of the mask prevents the gap between the mask and the substrate from being consistently regulated to a desired gap, and thus, enlarges the difference in the dimension of the pattern transferred to the substrate. In a region where the gap is larger than the desired gap, for example, the width of lines transferred to the substrate are undesirably larger than the desired width, and vice versa. As a result, the width of lines undesirably varies widely on the substrate.
To correct for the undesirable deflection of the mask, the deflection of the mask needs to be measured or determined. Accordingly, a need exists to provide a way for determining the deflection of the mask.